1. Field of the Invention
The invention relates to liquid purification and separation, and also to fertilizer chemistry. More specifically, the invention involves the conversion of activated sludge into a manageable material suited for use as fertilizer, or the conversion of organic brewery waste into cattle feed. The process of the invention also produces a carbonaceous material that can be adapted for use as insulation, soil conditioner, or carbon black.
2. Description of the Prior Art
Modern sewage treatment involves the aeration of sewage in the presence of aerobic bacteria, which convert the sewage into a gelatinous mass known as activated sludge. The sludge settles in large tanks and eventually is drawn off in a form that contains about 99% water. Various treatments are applied at this point to further dehydrate the sludge, including the addition of sulfuric acid and heat to cause evolution of gas, which raises the solids from the liquid and reduces the water content of the sludge to about 85%. This process is described in U.S. Pat. No. 1,025,622 to Grossmann, U.S. Pat. No. 1,325,787 to Grossman, and U.S. Pat. No. 1,430,182 to Peck.
Chemical and mechanical means for dehydrating sludge are known to include the addition of flocculants to the sludge and the use of mechanical vacuum drums to physically draw some of the liquor from the sludge. The dehydration achieved by these means reduces the volume of solid mass but does not alter the gelatinous nature of the remaining sludge. Even when the sludge has been substantially dehydrated, it remains unmanageable because of the physical characteristics of the end product.
Attempts to eliminate the sludge disposal problem are reflected in U.S. Pat. No. 3,922,221 to Rosler and U.S. Pat. No. 3,948,774 to Lindman. Each of these patents requires a specialized plant structure in order to chemically and physically process the sludge into water and a solid residue. The high cost of installing and maintaining such systems makes use practical only in specialized situations where sewage storage is not possible.
Sludge burning, known as Pyrolysis, was thought to be an ideal solution to the problem of sludge disposal.
The sludge problem has worsened to crisis proportions since 1972, when Congress passed legislation requiring municipalities to upgrade sewage treatment facilities as an aid to reducing water pollution. With the help of federal funds, sewage treatment plants have improved their ability to extract sludge from sewage through secondary treatment. One expert has estimated that the elevation of a sewage treatment plant facilities from primary to secondary treatment can mean from fifty to one thousand percent increase in the facility's sludge output. Accordingly, the disposal problem has grown as the quantity of sewage treatment has increased. Current estimates are that the United States produces three hundred million tons of wet sludge per year, with the Environmental Protection Agency estimating that sludge disposal costs may be in excess of one billion dollars annually. The problem and the costs are expected to substantially increase in 1977, which is the deadline for fully secondary treatment of sewage by all communities.
Sludge disposal is limited in that the sludge can be placed only in the air, in the sea, or on land. Air disposal, by burning, was thought to be an ideal solution and a burning process known as pyrolysis was developed to produce charcoal as a by-product. However, after the oil embargo of 1973, burning became overly expensive and the problem of air pollution further added to the demise of the burning process.
Disposal in the sea has long been the practice in coastal areas, but in recent years both federal and state concern for this practice has increased. The federal government has ordered both New York and Philadelphia to phase out their dumping in the Atlantic, and California has ordered the City and County of Los Angeles to stop disposal in the Pacific. The problems of ocean dumping are that marine life is subjected to unknown hazards and, at least on the Atlantic coast, the sludge has returned to beaches and presented severe pollution problems.
Disposal on land is the presently preferred approach, but problems exist because of nature and composition of the sludge. Activated sludge is composed primarily of human waste and contains bacteria and viruses that can be harmful to humans and animals. In addition, some sludges contain heavy metals discharged by industry. Thus, it is important that sludge does not enter drinking water supplies or contaminate crops. One of the unsolved problems of disposal on land is that the sludge is an unmanageable gelatenous material that is difficult to penetrate with conventional sterilizing agents. When the gel is dumped as land fill, it retains water for exceptionally long periods of time, produces offensive odor, and is a breeding ground for harmful bacteria and viruses that can enter ground waters through rain water run-off.
Sludge is known to contain beneficial nutrient for plant growth and could be used as a fertilizer as long as heavy metal release is controlled. The main problems in this use are that sludge is difficult to transport in gelatenous form and may not be evenly spread as conventional dry fertilizers. The odor of sludge fertilizers is also a severe problem, and this problem is also complicated by the gelatenous nature of the sludge, which resists treatment by deodorizing agents.
A method of treating organic garbage that may include sewage and that produces a compost-like material is disclosed in U.S. Pat. No. 3,758,287 to Scheel. This patent suggests combining highly concentrated sulfuric acid with magnesium oxide and organic waste and allowing the mixture to exothermically react, the resulting product being a sterile compost. This process creates a problem in the soil because of the presence of magnesium sulfate, Mg SO.sub.4, in the compost. Magnesium salts of strong acids tend to make the soil overly acidic because, by cation exchange, absorbed hydrogen is replaced by metallic cations of the salt, liberating a strong acid in the soil, for example H.sub.2 SO.sub.4. Therefore, salts such as MgSO.sub.4 and MgCl.sub.2 are seldom used in lime materials, which more common agricultural limes may include magnesium salts of weak acids, such as carbonates, or basic compounds such as oxides and hydroxides, which leave no objectionable residue in the soil.
The terms "sludge" and "sewer sludge" as used hereafter shall refer to sludges that have been dehydrated to a gelatinous state, either through known means as have been described above or through means yet to be devised.